Crystal Engineering of Chelating Hybrid Ultramicroporous Materials via Pillar Modulation for Energy-Efficient Acetylene Separation

Acetylene, C₂H_2, a commodity chemical, is a building block for producing plastics, synthetic rubbers, and pharmaceuticals. Among emerging classes of recyclable adsorbents, hybrid ultramicroporous materials (HUMs) demonstrate C₂H₂-selective physisorption, yet their C₂H₂ over CO₂ (C₂H₂/CO₂) separation performances are often constrained by trade-offs between uptake capacity, selectivity, ease in regeneration, and stability under humid conditions. Expanding upon the prototypal chelating ligand N¹,N²-bis(pyridine-4-ylmethyl)ethane-1,2-diamine (enmepy)-derived HUM family [Zn(enmepy)(MF₆)]_n, MFSIX-enmepy-Zn, through systematic pillar modulation, three previously unreported HUMs are isolated as single crystals. [Zn(enmepy)(SnF₆)]_n, SNIFSIX-enmepy-Zn, offers limited stability, whereas [Zn(enmepy)(TiF₆)]_n, TIFSIX-enmepy-Zn, and [Zn(enmepy)(NbOF₅)]_n, NbOFFIVE-enmepy-Zn are more robust. Gas sorption isotherms, sorption kinetics, dynamic column breakthrough experiments, and molecular modelling identify NbOFFIVE-enmepy-Zn as offering a combination of high C₂H₂/CO₂ selectivity (>5) with low regeneration energy requirements (≈31 kJ mol⁻¹) and superior hydrolytic stability (>7 days, 75% relative humidity). Further, NbOFFIVE-enmepy-Zn delivers effective C₂H₂/CO₂ separation (a separation factor > 5) under both dry and humid conditions, underscoring the importance of fine-tuning inorganic pillars in balancing adsorption performance and material robustness.